In our Laboratory we have continued the work in developing new instruments using electronics, optics and ultrasonics. In 1996 a record number of 4 PhD's were graduated.
We have developed a simple and robust ice detection sensor which initially was intended to be used on airplane wings. Another interesting application is ice formation in deep-freezer cabinets where the ice layer seriously consumes excess electricity. Yet another application is the so-called Very Cold-Project where automatic weather stations will be equipped with ice sensors developed by our laboratory. The work has been done in co-operation with VTT and Labko.
Yet another succes has been the development of a tension meter for optical fiber production. The initial academic research was carried out in our laboratory and it is now utilized routinely by fiber-optics industry in the manufacture of optical fiber increasing the productivity substancially.
Due to the general economic conditions, applications for more outside funding have been increased. The problem is the long time lag between the application date and the final outcome of the application.
FIBER OPTIC DIGITAL CONFOCAL MICROSCOPE
Pekka Raatikainen, I. Kassamakov*, Peeter Paris** and M. Luukkala
We have developed further the fiber optic confocal microscope in such a way that the vibration sensitivity is minimized as the whole of the microscope is inside a single-mode optical fiber and the light source is a visible semiconductor laser. The depth r esolution is considerably better than with a conventional optical microscope.
* Bulgarian Academy of Science, Institute of Applied Physics, Bulgaria
** University of Tarto, Estonia
THERMAL WAVE INVESTIGATION OF THE SURFACE QUALITY OF COPY PAPER
Otto Karjalainen and M. Luukkala
In co-operation with KCL (Paper Science Center) we have developed a new instrument for evaluating the surface quality of photocopying paper. The surface properties are one factor in determining the quality of the copy print on paper.
THERMAL NONDESTRUCTIVE EVALUATION OF SPACE TELESCOPE MATERIAL
Reijo Lehtiniemi*, Jussi Varis and M. Luukkala
The X-ray telescope tube for the European XMM science satellite is being built in Finland by Finavicomp Oy, and will be launched into the space in 1999 by the new Ariadne 5 carrier rocket.
Since the requirements for the tube are extremely high, it is constructed of unidirectional carbon fiber composite material which allows one-dimensional curvature and maximal strength with minimal weight. Since one kg of the material costs ~ 100,000 FIM and in space there is no second chance, the need for thorough testing of the tube is obvious. Broken fibers forming vertical cracks may produce serious problems. Unfortunately, the carbon fiber composites have traditionally been very difficult to be tested nondestructively but the line-scanning thermal method developed in the Electronics Research Laboratory can be applied directly. Usual thermal methods generate only vertical heat flow in the sample, but because of the lateral heat flow component produced by our method, also vertical defects can be detected.
The manufacturer's Standard Reference samples were inspected with the method, and the artificial test defects were detected. So far our method has provided the best results of all possible nondestructive testing methods.
Present address: Nokia R & D, Helsinki
DETERMINATION OF GLAZE THICKNESS ON CERAMIC SUBSTRATE USING AN INFRARED CAMERA
Jussi Varis, A. Nurminen* and J. Tuominen*
Ceramic objects coated with polished glaze are very common in every household. Nowadays, the manufacturing processes of these items are partly automated, and for instance, the spraying of the glaze on a ceramic substrate is done with an industrial robot. To reduce raw material waste, the thickness of the glaze has to be measured in a non-destructive way. The ceramic substrate is usually hardened in its mold in a furnace. The substrate is then moved from the furnace via processing line to the industrial robot, which sprays the substrate surface with semi-liquid glaze. During the spraying the semi-liquid glaze is much cooler than the still warm substrate and as a result the heat from the substrate transfers through the glaze.
The thickness of glaze on a warm ceramic substrate was determined by measuring the surface temperature behaviour of the glaze with an infrared camera. A one-dimensional theoretical model was developed for describing the heat conduction in the glaze. Based on curve fitting to the temperature data, estimates of the glaze thicknesses were obtained. For comparison, the thicknesses were also measured destructively.
Department of Computer Science, Helsinki University of Technology, Espoo, Finland
THERMAL LINE-SCANNING NONDESTRUCTIVE EVALUATION
Reijo Lehtiniemi*, Jussi Varis and M. Luukkala
The thermal line-scanning nondestructive evaluation has been studied and a flexible, multipurpose system for real-life testing has been constructed. The concept is based on the localized linear heating of the sample under inspection and simultaneous line-scanning infrared thermographic monitoring of the thermal response of the sample, affected by the possible defects in the sample structure. Thermal images of the sample, revealing also sub-surface irregularities, can be recorded by scanning the sample pependicularly to the together parallel heating and monitoring lines.
Compared with more traditional area heating techniques, the linear geometry of the system enables utilizing a larger selection of heating options of more linear nature. Some of these offer extremely interesting advantages from nondestructive evaluation point of view, such as the high localizability and power density of laser and selectiveness of inductive heating.
To offer the best possible performance for the maximum number of applications, the system has been designed modular so that different testing combinations are easy to configure. Applying theoretical analysis and numerical simulation package is an essential part of the entire testing process, so that the assembly of the system can be optimized for each individual problem.
The system has been applied for example to practical nondestructive testing of plasma-spray coated nuclear power plant components and carbon glassfiber aero space composite materials, which have traditionally been extremely difficult or even impossible to test without damaging the specimen. The system is available both as a laboratory version for better accuracy and as a transportable version for on-site testing.
* Present address: Nokia R & D, Helsinki
MONITORING MICROBIOLOGICAL GROWTH IN PREMADE FOOD PACKAGES FOR END-TESTING APPLICATIONS
Edward Hæggström and M. Luukkala
This EUREKA-project has participants from Holland, France, Sweden and
Finland. We have reached a niveau where we are able to detect certain contaminants
at quite low levels of contamination after a comparatively short incubation
time particu\- larly in packaged milk. The semi-automatized system is based
on an ultrasonic interrogation idea.
DEVELOPMENT OF THE ICE DETECTION SENSOR
In this project we have applied the sensor to detect ice formation in
very cold conditions, for example in wind turbine blades in Lapland in
a successful way. This work is done in co-operation with VTT and Labko.
OPTICAL DETECTION TECHNIQUE FOR PHOTOACOUSTICALLY GENERATED SURFACE ACOUSTIC WAVES
Juha Aaltonen, Jussi Varis and Jyrki Stor-Pellinen
Photoacoustic techniques, where sound in a material is produced by pulsed laser-light excitation, show great promise for solving various industrial problems like paper quality monitoring etc. The main advantage is the non-contact ultrasound generation in the studied material though ultrasound detection has been contacting so far. Hence non-contacting detection methods, like air ultrasound techniques, have been developed. Optical techniques, for instance interferometers, are also possible and they work especially well with highly reflective metals. Hence, a surface displacement technique has been studied. This method is simpler and more robust than an interferometer and thus more applicable in harsh industrial conditions.
A NONCONTACT THICKNESS MEASUREMENT OF THIN SAMPLES AND PAPER
To measure thickness of a thin film or a foil without touching the sample in real time is essential in many industrial applications. Normally small pieces of transparent samples are taken under a microscope where the thickness can be estimated. Interferometric systems are also available but they are expensive. Paper industry needs information of the profile of the paper during manufacturing process. Measuring area in paper mills is very large, so the measurement has to be fast.
A new reliable and fast ultrasonic method using 20 kHz to 40 kHz ultrasound and 7 kHz to 15 kHz pulsed sound has been studied. A wide range of thin samples have been tested from 2 um Mylar to 100 um paper with good accuracy. This system can also be used with opaque samples. The optimum sound frequency is chosen depending on sample thickness. The more massive the sample the lower the frequency. Both electrostatic and piezo ceramic transducers have been used and both have advantages and disadvantages and can only be used within narrow bandwidths. The sound pulse system consists of a loudspeaker and a pair of microphones.
MAGNETOACOUSTIC MEMORY PHENOMENON
Jyrki Stor-Pellinen, V. Ermolov* and M. Luukkala
The majority of modern signal recording is based on the preservation of the state of magnetization in ferromagnetic films or tapes as a local magnetization distribution. In magnetic materials, the magnetic and acoustic phenomina are connected by magnetostriction. Simultaneous exposure of a magnetic medium to mechanical stress and a magnetic field changes the magnetic domain structure. In several magnetic solids, there is a so-called long-term acoustic wave memory which appears as an elastic response when suitable material is subjected to a magnetic reading pulse, if there was a prior elastic signal to the memory. The phenomenon has been observed in a wide variety of materials.
We are studying the magnetoacoustic memory phenomenon in ferrites and applications of the phenomenon to signal processing by the developement of a tool we call the magnetoacoustic memory correlator. The correlator is very promising in view to applications as it operates in real time: the correlation is available once the two signals have been written.
The correlator utilizes the nonlinear interaction of the magnetic and acoustic wave in its medium. An acoustic reading signal applied to the correlator with a stored magnetization distribution generates a magnetic field that is thei r convolution, that for its part is proportional to the correlation between the stored signal and the reading signal. We have tested the correlator in 2.5 MHz to 50 MHz NDT applications for a better signal-to-noise ratio.
* Moscow Engineering Physics Institute Electronics Research Electronics Research Department of Physics, Annual Report 1992